The present invention relates to a microwave heating apparatus and, more particularly, to a microwave heating apparatus which utilizes the surface-wave mode of microwave propagation.
A microwave heating apparatus utilizing the surface-wave mode of microwave propagation is known as having various features because high frequency electromagnetic energy (microwave energy generated by a high frequency electromagnetic wave generator or a magnetron tends to be centered on the surface of a surface-wave transmission line. Some of these features, which can be appreciated when a dielectric material to be heated or heat-treated, such as a foodstuff or a web of cloth, is subjected to high frequency electromagnetic surface-waves by placing it on or to the surface of the surface-wave transmission line, are that the dielectric material can readily be heated in a relatively highly efficient manner, that the dielectric material, in the case of a foodstuff, can be browned or scorched which cannot otherwise be achieved by a known microwave oven, that a substantially uniform distribution of high frequency electromagnetic waves can be obtained and that a device, and its associated accessories, for preventing the leakage of microwave, energy which is to be incorporated in the microwave heating apparatus of the type referred to above, can be simplified.
The surface wave transmission mode is known as one of the modes of propagation of high frequency electromagnetic energy and is largely utilized in various fields of microwave engineering and, particularly, in a wave guide duct or a transmission line. The microwave surface transmission has the following properties.
First, the intensity of surface waves generated by the high frequency electromagnetic wave generator tends to exponentially decrease as the surface waves travel away from the surface of the transmission system in a direction perpendicular to the direction of transmission of the surface waves and, therefore, no electromagnetic power is propagated nor radiated in this direction.
Secondly, the surface waves travel at a phase velocity smaller than the velocity of travel of a light beam.
It is well known that, as a circuit for surface transmission of microwaves, a dielectric flat-surfaced structure or a corrugated electroconductive plate has been employed. Various attempts have heretofore been proposed to apply the microwaves travelling along a surface structure to a heating apparatus, one type of which attempts is disclosed in the U.S. Pat. No. 3,478,187, patented on Nov. 11, 1969, and it has been found that all of these attempts are directed to utilization of the first mentioned property and that these attempts can successfully be practiced so far as continuous drying of a sheet-like material such as a film or a cloth is concerned. However, the conventional method of heating the dielectric material by the use of the surface wave mode of microwave propagation has the following disadvantages because of the nature of the arrangement of a surface wave transmission circuit which will now be described with particular reference to FIG. 1, which shows a schematic sectional view of a corrugated metallic surface for the purpose of explanation of the principle of the circuit arrangement.
Referring to FIG. 1, the corrugated metallic surface forming a surface transmission circuit is designated by C and is coupled to a source of high frequency electromagnetic waves B through a coupling circuit G. When the source of high frequency electromagnetic waves B, such as a magnetron, is operated, microwaves generated thereby travel towards and along the corrugated surface. During the operation of the magnetron B, corrugations on the metallic surface C, each pair of the adjacent members of which corrugations defines a stub A, are electromagnetically coupled to each other by means of the respective stubs A at a portion adjacent the crest or top of the corrugations.
In the heating apparatus of the type employing the corrugated metallic surface referred to above, since a material to be heated D is adapted to be placed on the corrugated metallic surface bridging over some or all of the stubs A and among some or all of the corrugations, the condition in which the corrugations are electromagnetically coupled to each other considerably varies as the condition of the material to be heated D varies. More specifically, high frequency electromagnetic energy derived from high frequency electromagnetic waves travelling along the surface and over the stubs A tends to be fed towards the left of the drawing of FIG. 1 sequentially propagating over the respective stubs A. However, the presence of the material to be heated D on the corrugated surface causes a substantially complete loss of the high frequency electromagnetic energy at a position rearwardly of the material to be heated D with respect to the direction of transmission of the high frequency electromagnetic surface travelling waves. This means that the material to be heated D is heat-treated in such a manner that a portion of material D which faces the magnetron B tends to be extensively heated while the opposite portion of the same material D remote from the magnetron B tends to be underheated, and that oscillation of the surface waves tends to be disturbed especially at a portion occupied by the material D and a portion rearwardly of the material D remote from the magnetron B.
According to a series of experiments conducted by the inventors, it has been found that, in the case where water is employed fr the material to be heated, the surface wave mode is disturbed at a position rearwardly of a glass cup with the water and with the high frequency electromagnetic energy available at that position being reduced to about one-fifth of that available at a position on the other side of the water-filled cup which is close to the magnetron. Accordingly, in the case where two or more separate materials to be simultaneously heated are arranged one behind the other in a direction parallel to the direction of travel of the wave front, they cannot be uniformly heated. In view of this, such a heating apparatus as hereinbefore described cannot be used as a household kitchen utensil, as it will not exhibit the required performance, because the shape and type of individual materials to be heated are not fixed.
For the reason described above, the surface wave transmission circuit in the form of the corrugated metallic surface of the construction shown in FIG. 1 has the following disadvantages:
1. Because of the complicated shape, the corrugated surface cannot easily be manufactured without difficulty. PA1 2. Since the space wherein the surface waves are propagated and the space in which a material to be heated are the same, the device cannot accommodate changes of the shape and type of material to be heated. PA1 3. The device cannot be utilized other than for continuously drying a cloth or like sheet material to be heated. In other words, where a material to be heated, which has a relatively great thickness, is heated solely relying on the surface waves, since the surface wave distribution tends to exponentially decrease as the surface waves travel away from the surface of the transmission system as hereinbefore described, a sufficient and required amount of high frequency electromagnetic energy will not penetrate deep into the material to be heated. Consequently, there is a disadvantage that only a portion of the material which faces the surface is heated and, therefore, the device cannot be utilized other than in continuous drying of a sheet-like material or material having a relatively small thickness.
There is known another type of microwave heating apparatus utilizing a surface wave transmission circuit in the form of a wave-guide. As shown in FIG. 4 of the accompanying drawings, the waveguide WG has one wall member E formed with a plurality of equally spaced slits S and, therefore, is generally referred to as a ladder type surface wave transmission line. In this apparatus, since a sheet-like material, for example, a film, to be heated is continuously fed along the outside of the wall member E in such a manner as to cover the equally spaced slits S while high frequency electromagnetic energy, which oscillates the surface waves, is guided within the interior F of the waveguide WG, the second mentioned disadvantage described above can be substantially eliminated. However, in the above described apparatus having the waveguide of the construction shown in FIG. 4, no substantial measure are taken to protect the human or user from being exposed to the high frequency electromagnetic energy radiated to the outside of the apparatus. Moreover, the apparatus utilizing the waveguide involves the third mentioned disadvantage described above.